Liquid-liquid extraction is used generally in metal separation processes, allowing metals to be extracted from aqueous solution using organic extraction solutions. An extraction solution consists of an extraction reagent and a hydrocarbon solvent. The extraction reagent is generally diluted in a hydrocarbon solvent, which dissolves into an aqueous solution or evaporates into air as little as possible in process conditions.
The composition of both the active extraction reagent and its hydrocarbon solvent in the extraction solution has been found to change during long-term industrial use. As a result, the metal-binding power of some extraction reagents may have worsened. In particular this has been observed in copper extraction processes and nickel extraction processes which utilize various reagents based on hydroxyoxime derivatives. These reagents are also used for the extraction of certain other metals and metalloids (e.g. palladium and germanium) as well as in some synergistic extraction reagent mixtures to modify the selectivity for different metals.
It is known that a hydroxyoxime reagent used in extraction degrades in the hydrolysis reaction into aldehyde or ketone and may be reoximated using hydroxylamine (NH2OH) or a salt thereof. The reoximation reaction takes place as follows:R1R2C═O(ketone)+NH2OH→R1R2C═N—OH(ketoxime)+H2O
If R2═H in the formula, the source material in question is some aldehyde and the product the corresponding aldoxime. If R2 is for example an alkyl or aryl group, it concerns a ketone and ketoxime. This same equilibrium reaction from right to left, in other words acid-catalysed hydrolysis, is one of the decomposition reactions that occur when hydroxyoxime is used as the extraction reagent in the extraction process. However, it is known that hydroxyoximes also decompose e.g. in oxidation reactions.
The effectiveness of the hydroxyoxime reagent present in the organic phase is a crucial factor in the liquid-liquid extraction. Replacement of the organic phase extraction reagents would require significant costs as well as reduced efficiency in the process due to shut down of the process during replacement.
US2012/0080382 describes a method and apparatus for restoring the extractive potential of organic hydroxyoxime-based extraction solution used in the recovery of metals by liquid-liquid extraction. The method is two-stage, in which an aqueous solution of hydroxylamine or some hydroxylamine compound is used in the reaction stage, and the removal of the undesirable compounds generated in the reaction occurs in the second stage by adsorption purification. The reaction stage and the adsorptive stage are carried out in a mixing tank. This liquid-liquid method has been effective for the regeneration of degraded aldoxime type extractants but for ketoxime type reagent it is slow and the conversion remains low after treatment.
Several processes of extraction of metals from aqueous solutions containing metal values in which the degraded circuit organic phase (extractant phase) is reoximated to again restore the effectiveness of the oxime extractant present in the organic phase have been disclosed in the prior art. U.S. Pat. No. 4,104,359 discloses use of solid-liquid reaction mixture in a process for regenerating an active component of an organic extractant. It teaches that organic sulphonic acid causes the degradation of a α-hydroxyoxime reagent in the organic phase and that α-hydroxyoxime can be reoximated in a solid-liquid reaction mixture directly using a solid hydroxylamine salt. The method can also be used for β-hydroxyoximes, which are ketoximes. Said patent mentions that alternatively a saturated aqueous solution of hydroxylamine may be used and the process may be performed for instance in a mixer-settler-type of extraction cell. The method emphasizes percolation of degraded extractant solution past only excess amounts of solid hydroxylamine acid salt as a technique to accomplish the regeneration reaction. The method uses only excess amounts of hydroxylamine salt in absence of an alkali.
U.S. Pat. No. 5,993,757 relates to an improvement in the process of extraction of a metal, the improvement comprising the reoximation of decomposed hydroxyoxime extractant using hydroxylamine salt. In this method distillation will most typically be employed on the organic phase that is recovered from raffinate by coalescers or by skimming it off from ponds. Furthermore, the reaction of the ketone and aldehyde degradant with hydroxylamine will be carried out in the presence of a catalytic amount of a phase transfer catalyst.
U.S. Pat. No. 5,300,689 discloses a process for oximation of carbonyl compounds, such as ketones and aldehydes by oximation with hydroxylamine including oximation processes carried out in the presence of a catalytic amount of an acid phase transfer catalyst, such as 2-ethylhexanoic acid, and/or the presence of an alkali metal or alkaline earth metal catalyst.
Regeneration of degraded oxime extractant in process organic solution has also been disclosed in AU1989044515. However, this method employs a liquid-liquid reaction in presence of aqueous ammonium hydroxide.
The regeneration behaviour of process organic solutions tend to vary from industrial solvent extraction (SX) process to another due to the differences in chemical and physical properties of process solutions and operational practices, among others. In some cases the regeneration reaction can be done in a shorter period of time whereas in others it may require relatively longer residence times. There is a need for an economical and effective process for restoring the extractive potential of organic hydroxyoxime-based extraction solution used in the recovery of metals by liquid-liquid extraction.